At present, there are principally used two different processes to cast a foaming mixture of polyurethane in a textile cover previously disposed in a mold.
A first known process consists in first applying to the textile a fluid type barrier constituted by a rubber, polyurethane or like film, preventing the foaming mixture from penetrating and passing through the mesh of the textile.
Moreover, the fluid-tight film permits maintaining the cover in the mold by suction. But the film is not structurally modified during ultimate heating to cure the foam.
The principal drawback of this process is therefore that, due to the fluid tightness of the textile cushion cover resulting from this film, a large part of the comfort of the cushion is lost, particularly because of perspiration from the user.
The second known process consists in applying to the textile, as a barrier, a sheet of flexible polyurethane foam previously cut off from a block of the foam.
But this process is not satisfactory unless the foam sheet is of microcellular structure, that is very little permeable to air thanks to the cells which are substantially closed and substantially do not communicate with each other. Thus, in this case, the foaming mixture does not penetrate too greatly through the cells of this foam sheet, thereby partially avoiding the effect of hardening, known as the "cardboard effect", resulting from densification of the foaming mixture. Nevertheless, the permeability of the foam sheet is extremely difficult to control during fabrication of the blocks, whereby a large portion of the production is defective in spite of all efforts because of too great permeability of the sheet of foam, which gives rise to too much penetration of the foaming mixture into the cells of the sheet of microcellular foam, resulting precisely, upon densification of the foaming mixture, in the formation of a crust called "cardboard effect", which is disagreeable to the user from a standpoint of comfort, and disastrous for the manufacturer from an economic standpoint because of the waste of a large part of the foaming mixture uselessly lost through densification and crust formation.
There can also be used, in place of a sheet of microcellular foam, a sheet of flexible foam which has first been hot calendered. But the same drawbacks persist with hardly any improvement.
Moreover, the maintenance of the textile covering within the mold obtained ideally by holding said covering against the internal wall of the mold by suction, is rendered impossible if the barrier is constituted by a sheet of microcellular foam or a sheet of calendared foam, the two being porous and hence permeable to air, negating the effect of suction produced for example by the generation of a vacuum.